Kaolin clay (kaolinite) is a naturally occurring, crystalline aluminosilicate material having the chemical formula Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4 and structurally consisting of linked, alternating layers of tetrahedral silicon and octahedral aluminum. Mined crude kaolin clay is typically refined for use as fillers in rubber, plastics, and other polymers, as well as for use as pigments or pigment extenders in paints and other industrial coatings. Kaolin clay crudes are generally processed in one of two ways: 1) via an airfloat process wherein the crude clay is crushed, dried, pulverized, and then air-classified to the desired particle size and to remove unwanted impurities; or 2) via a waterwashed process wherein the crude clay is dispersed in water, degritted, fractionated and then subjected to various chemical beneficiation steps to improve its brightness properties. The subsequent chemical treatment of such clays has typically been accomplished via the addition of the treatment additives, in neat or emulsified form, to an aqueous slurry of the dispersed clay or to the clay in dry powder form with good mixing where after the treated clay is dried as needed to yield a dry product of moisture content less than 1%. Alumina trihydrate (commonly referred to as ATH or gibbsite) has the chemical formula Al(OH).sub.3 and is typically refined for use as fillers or as flame retardants in plastics and other polymers. ATH fillers having specific particle size properties can be produced via direct precipitation methods or coarse particle precipitates of ATH can be mechanically ground by either wet or dry grinding methods to yield the desired products of finer particle size. The chemical treatment of ATH products has typically been accomplished via the addition of the neat treatment additives to the ATH in dry powder form with good blending in a solids/liquid blending device with the optional use of heat.
Although mineral additives like kaolin clay, treated kaolin clay, ATH or treated ATH have historically been viewed as merely low cost fillers, they are often a critical factor in the processing of polymeric composites. These processing aspects are particularly important in thermoset compounds, such as those prepared from epoxies or unsaturated polyesters, since mineral filler loadings are typically high (e.g., on the order of 80-120 phr or higher). Minor variations in filler properties are well known to cause significant variations in the paste viscosity profile of thermoset compounds. Hence, the physical properties of the fillers must be maintained within certain tightly controlled limits to produce thermoset composites of very consistent quality. Furthermore, having a lower viscosity filler can be important in thermoset applications in terms of improving the processability of the filled compound and/or for greater ease of dispersion of the filler. It should be noted that achieving improved filler dispersion very often results in improved physical properties for the finished plastic. In addition, lower compound viscosities permit increased loadings of mineral fillers (to decrease compound cost and/or increase certain physicals as desired) without loss of processability.
One way in which the wet-out, dispersion and resultant viscosity properties of a given mineral filler can be improved is through chemical treatment with additive(s) that make the filler's surface more organophilic and thereby more compatible with the polymer matrix. Various treatments of mineral fillers, such as the treatment of kaolin clays, for subsequent use in polymers are known in the prior art as will be later discussed. However, the specific organic functionality, polarity, hydrophobicity and cost associated with such chemical treatments can have a tremendous influence on the resultant performance and cost versus performance characteristics of the treated mineral product. This behavior is a function of the surface chemistry of the mineral filler, the chemistry of the polymer matrix and the end-use performance benefit desired. Some treatments for clay or ATH fillers with various silanes or blends of silanes are known. However, we have found that the use of silane treatments alone on clay often do not yield the amount of viscosity reduction in thermosets desired and are generally too expensive relative to the viscosity reduction benefits obtained. In addition, the silane treatment of clays or ATH can often cause other problems in terms of producing treated products so hydrophobic in nature that airborne dust hazards are created during their handling. Consequently there has been a long-felt need for a cost effective, but low dusting treatment for mineral fillers, such as for kaolin clay or ATH, that significantly reduces the paste viscosity of thermoset compounds containing the mineral fillers.